Conventional golf balls can be divided into two general groups: solid balls or wound balls. The difference in play characteristics resulting from these different types of construction can be quite significant. Balls having a solid construction are popular with golfers because they provide a very durable ball while also providing maximum distance. Solid balls are generally made with a solid core, usually made of a cross linked rubber, enclosed by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. In addition to one-piece solid cores, solid cores may also contain a number of outer layers, such as in a dual core golf ball. The cover is generally an ionomeric material, such as SURLYN®, which is a tradename for a family of ionomer resins produced by E.I. DuPont de Nemours & Co. of Wilmington, Del. Covers are typically a single layer but may also include one or more layers, such as in a double cover having an inner and outer cover layer.
The combination of the solid core and ionomeric cover materials provide a ball that is very durable and abrasion resistant. Further, such a combination tends to impart a high initial velocity to the ball, which results in increased distance. Because these materials are very rigid, however, solid balls can have a hard “feel” when struck with a club. Likewise, due to their construction, these balls tend to have a relatively low spin rate, which can provide greater distance and increases accuracy off the tee.
Wound balls typically have either a spherical solid rubber or liquid center, around which many yards of a tensioned elastomeric thread are wound. The wound core is then covered with a durable cover material, such as SURLYN® or similar material, or a softer cover material, such as polyurethane. Wound balls are generally softer and provide more spin, which enable a skilled golfer to have control over the ball's flight and landing position. Particularly, with approach shots onto the green, the high spin rate of soft, wound balls enable the golfer to stop the ball very near its landing position.
To make wound golf balls, manufacturers use winding machines to stretch the elastic threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. Generally, as the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more resilient wound ball is produced, which is desirable.
For wound golf balls, the thread is typically formed by a calendar and slitting method rather than an extrusion method. The calendared thread typically has a rectangular cross-section, while extruded thread generally has a circular cross-section.
Prior art wound golf balls and cores typically use polyisoprene rubber thread wound onto the cores at elongations of between 500 to 1000%. The amount of thread required for a golf ball core is dependent on the elastic modulus of the thread in the elongated state. Elongated polyisoprene thread generally has an elastic modulus of 10,000 psi to 20,000 psi. Further, the properties, in particular resilience, of the wound ball or core are dependent on how well the thread packs during winding. The dimensions of the thread and winding pattern control the packing density. Present art polyisoprene threads are typically at least 1/16 inches wide by 0.02 inches thick, measured prior to winding. Present art polyisoprene thread, however, is commonly produced in thicknesses between 0.014 inches and 0.024.
U.S. Pat. No. 6,149,535 discloses a thread for winding having at least about 10 individual strands that are each at most about 0.01 inches in diameter. Preferably, the thread has more than 25 strands with diameters of less than about 0.002 inches. The smaller thread dimension allows the thread to be wound more densely. Preferably, the elastic modulus of the thread is greater than 20 ksi when wound about a center. Preferably, the maximum elongation of the thread is greater than about 8%.
A variety of golf balls have been designed by manufacturers to provide a wide range of playing characteristics, such as compression, velocity, “feel,” and spin. In addition to ionomers, one of the most common polymers employed is polybutadiene and, more specifically, polybutadiene having a high cis-isomer concentration.
The use of a polybutadiene having a high cis-concentration results in a very resilient golf ball. These highly resilient golf balls have a relatively hard “feel” when struck by a club. Soft “feel” golf balls constructed with a high cis-polybutadiene may also be constructed, however, they tend to have low resilience. In an effort to provide improved golf balls, various other polybutadiene formulations have been prepared, as discussed below.
The most common polymers used by manufacturers to modify the properties of golf ball layers and/or covers have been ionomers, such as SURLYN, commercially available from E.I. DuPont de Nemours and Co., of Wilmington, Del. Recently, however, manufacturers have investigated the used of alternative polymers, such as polyurethane. For example, U.S. Pat. No. 6,132,324 is directed to a method of making a golf ball having a polyurethane cover. This patent is hereby incorporated by reference.
Polyurethanes have been recognized as useful materials for golf ball covers since about 1960. Polyurethane compositions are the product of a reaction between a curing agent and a polyurethane prepolymer, which is itself a product formed by a reaction between a polyol and a diisocyanate. The curing agents used previously are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Since 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a diamine. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol. The first commercially successful polyurethane covered golf ball was the Titleist® Professional® ball, first released in 1993. Subsequently, the Titleist® Pro-V1® ball was introduced successfully in 2000 with a solid resilient solid polybutadiene core, a hard ionomer casing and a polyurethane cover. The Pro-V1 ball provided both professional and amateur players with long distance off of drivers and control for greenside play.
There remains a need, however, for other hard casing polymers that perform similarly to the ionomer casing to give golf ball designers more flexibility.